The present invention relates to a manufacturing method and apparatus for making a single silicon crystal of large diameter by the Czochralski method hereafter call the CZ method).
A manufacturing method of a single silicon crystal by the CZ method has been heretofore performed and is recognized as an almost perfect technique.
The diameter of a crystal required in the LSI industry has been increasing year by year. Nowadays, a 6-inch crystal is used for the most advanced LSI chip. It is anticipated that, in the near future, a crystal with a 10-inch or larger diameter, e.g. 12 inches, will be required.
As is generally known, according to this technique, when a molten silicon raw material is placed in a crucible made of quartz and a seed crystal is pulled gradually while said seed crystal contacts this molten surface, crystal growth is performed along with solidification of the contacted surface, thus providing a columnar single silicon crystal.
There are two concepts for crystal growing by the CZ process use of a rotating crucible or a non-rotating crucible. Today all CZ silicon crystals for use in semiconductors is manufactured by the conventional CZ process where crucible is rotated in the direction opposite to that of the crystal and the silicon pool is heated mainly by an electric resistance side heater which surrounds the side wall of the crucible. A silicon single crystal with a diameter larger than 5 inches has never been produced by the process where the crucible was not rotated or was heated by methods other than that mentioned above, though many attempts have been made to do so. This is due to the fact that temperature distribution which is perfectly concentric to the growing crystal has not been obtained without crucible rotation and by other heating methods such as an induction heater surrounding the crucible and an electric resistance heater placed under the crucible. It is important to note that crystal growing of this type is very sensitive to temperature.
In the conventional CZ process, the combination of a rotating crucible and an electric resistance side heater surrounding the crucible forces molten silicon liquid to by agitated strongly by convection. Accordingly, a temperature distribution which is uniform and perfectly concentric to the silicon crystal is obtained on the molten silicon surface, which is suitable for large diameter crystal growth. Therefore, the present invention is based on the conventional CZ process.
There is a big difference in liquid flow between the conventional CZ process and the other CZ process. This difference results in a big difference in conditions for crystal growing. The concept for crystal growing and the conditions involved therein had to be changed completely. The actions and structure of parts in the furnace also differed very much.
In this case, in order to make the single silicon crystal a P-type or N-type semiconductor according to the object, a proper quantity of doping material such as boron, antimony and phosphorus is mixed in the molten raw material However, the way of introducing these doping materials into the single silicon crystal is not fixed, and the concentration gets higher toward the lower part.
Furthermore, impurities such as oxygen and carbon that are mixed inevitably in production have a big effect in addition to doping materials that are intentionally mixed in the single silicon crystal as described above. That is, it is possible to improve the characteristic and the yield of a semiconductor by oxygen taken into the single silicon crystal. Therefore, it is desirable that oxygen is contained uniformly from the upper part to the lower part of the single silicon crystal, but the upper part has a higher concentration in general. Thus, the single silicon crystal is manufactured with the upper part of single silicon crystal having a low doping material concentration and a high oxygen concentration as the reference.
However, as the pulling of the single silicon crystal proceeds, the liquid surface of the molten raw material in the crucible is lowered, and the temperature of the molten liquid surface is changed. Therefore, the concentration of doping material is elevated and the concentration of oxygen is lowered in the molten raw material in the crucible Accordingly, the doping material existing in the single silicon crystal which is made to grow by pulling is increased gradually and oxygen is reduced, thereby causing the quality of manufactured single silicon crystal to fluctuate along the pulling direction.
Due to such variation of doping material and oxygen, the yield of usable wafers sometimes becomes as low as 50% or less in the case of a strict specification of components.
As an effective method of solving such a problem, a method is well known, where the liquid surface of molten raw materials is kept constant, resulting from the continuous feed of the silicon raw materials to the crucible. Some inventions about the method of pulling the single silicon crystal have been disclosed, for instance, in Laid-Open Patent Publication Nos. 84397/81, 88896/81, 164097/81, 36997/83, 130195/83, 241889/87 and 36197/86, and Laid-Open Utility Model Publication No. 141578/84. A brief discussion of these references follows.
In Laid-Open Publication Nos. 84397/81, 88896/81 and 164097/81, the crucible is not rotated. Moreover, induction heating is applied in 88096/81 and 164097/81. In 84397/81, the heating method is not disclosed. The method of using an electric resistance side heater surrounding crucible is not applicable to this method. In these methods, a silicon single crystal with a large diameter cannot be produced.
No. 36997/83 is concerned with oxide materials, and not silicon. Therefore, the crystal diameter required is small and the crucible is made of a noble metal with a high melting temperature. Therefore, induction heating, which is simple, is used in this case. A large diameter silicon crystal has never been produced by induction heating as mentioned above. This patent therefore does not meet the requirements of the present invention.
In No. 36197/86 and Utility Model No. 141578/84, an electric resistance heater is placed under the crucible in addition to side heater and is used as a main heater (In No. 141578/84, it is supposed to be used primarily. This is unfavorable for large diameter crystal growth. These two inventions are therefore unacceptable as crystal growing methods for use in LSI.
Moreover, in No. 36197/86, large heat radiation loss from partition results in the occurrence of solidification on the side wall of the partition in crystal growing area, as pointed out in No. 241889/87 (Page 2 line 12 to line 16, i.e., problems to be solved by this invention). Once the solidification starts, solidified silicon grows toward the crystal until touching it, thus interrupting the crystal growing operation. There are additional faults in this referenced invention. The holes in the dam (partition) which connect the raw materials melting area and the crystal pulling area are designed to be so large that the liquid silicon in both areas can flow in both directions, that is, from the outside to the inside and vice versa. Therefore, the liquid temperature outside the partition is decreased and the raw materials may not be molten in sufficient quantity for large diameter crystal growth, even though the heat insulating cover is disposed above the liquid outside the partition.
In Utility Model No. 141578/84, there exists the same problems as in the case of No. 36197/86, but the situation appears to be worse than in No. 36197/86.
No. 130195/83 and No. 241889/87 are based on the conventional CZ process. However, these prior art methods are suitable for large diameter crystal growth only conceptually.
No. 130195/83 does not solve the solidification problem on the partition wall which No. 36197/86 and Utility Model No. 141578/84 suffer from. Furthermore, this prior art invention cannot melt the raw materials at a rate which is required in a continuous charge CZ process for a large diameter crystal. The reasons are as follows: the feeding pipe of raw materials is immersed in the liquid silicon. Thus, the melting zone of raw materials is restricted within the narrow feeding pipe. As the raw materials cannot melt instantaneously, they are accumulated in the feeding pipe in the solid state.
No. 241889/87 may solve the solidification problem on the partition wall because there is no partition in the crucible according to this invention. But this prior art invention suffers from a huge problem concerned with the melting of raw materials. That is, the melting zone of raw materials is restricted to a narrow region as in the case of No. 130195/83. Therefore, this invention also cannot melt the raw materials at a rate which is required in a continuous charge CZ process. In addition, very expensive reformation of quartz crucible is required, which causes an increase in the production cost of a single crystal.
Recently, it has become possible to manufacture high quality granular polycrystalline silicon, and it is considered comparatively easy to feed such granular silicon to a molten raw material continuously and fixed quantity as disclosed in Provisional Publication No. 172289/83. However, when granular silicon is dropped on the liquid surface of the molten raw material, solidification is commenced with this granular silicon as a starting point. Therefore, it is impossible theoretically to grow a single silicon crystal by supplying granular silicon continuously by this method. The reason why solidification is commenced at dropped granular silicon is:
(a) the liquid temperature at the time of pulling a single silicon crystal is right above the melting point as apparent from the principle thereof, PA0 (b) since the specific gravity of silicon is lighter in a solid form than in a liquid form, granular silicon floats on the liquid surface, and PA0 (c) the emissivity of silicon is higher in a solid form than in a liquid form. PA0 (1) While pulling a single silicon crystal, the molten liquid temperature is fairly close to the melting point of silicon, but if granular silicon at a temperature close to the room temperature is fed continuously under such condition, granular silicon is not molten completely, but floats on the molten liquid surface as it is solid state, and the molten liquid is solidified and grown with the solid as a core. PA0 (2) When the melting portion and the single crystal pulling portion of granular silicon are partitioned off each other, solidification is liable to be generated from this partitioning portion because of the fin effect so called in electric heating and the higher emissivity than the molten silicon liquid, and if solidification is generated once, the growth thereof is continued and upbringing of a sound single silicon crystal is impeded. PA0 (3) When granular silicon is fed into a crucible for single crystal pulling by dropping continuously, a wave pattern is generated at the drop portion on the molten liquid surface, and the wave reaches the single silicon crystal pulling portion, thus impeding upbringing of a sound single silicon crystal. PA0 (1) a manufacturing method of a single silicon crystal for manufacturing a columnar single silicon crystal by pulling a molten raw material placed in a crucible, wherein: the inside of said crucible is partitioned off so that said pulled single silicon crystal is surrounded and said molten raw material may be moved; the molten liquid surface on the inside of said partition is maintained almost at a constant level with the whole molten liquid surface on the outside of said partition as granular silicon soluble area by feeding granular silicon to the molten liquid surface on the outside of said partition; said partition and the molten liquid surface on the outside of said partition are covered with a heat reserving board; and the temperature of the molten liquid on the outside of said partition is set higher than the temperature of the inside molten liquid at least by 10.degree. C. or higher, and provides further for executing the above-mentioned method, PA0 (2) a manufacturing equipment of a single silicon crystal comprising: a partition ring provided with small holes penetrated therethrough and immersed in said crucible so as to surround said pulled single silicon crystal; a heat keeping board disposed so as to cover said partition ring and the molten raw material on the outside of the partition ring; and a granular silicon feeding device disposed on the molten raw material on the outside of said partition ring, PA0 (3) according to above-mentioned equipment, a manufacturing equipment of a single silicon crystal, wherein a graphite crucible is partitioned off with heat resisting material of low thermal conductivity at locations corresponding to the partition rings, and heat insulting blocks are disposed underneath said graphite crucible, PA0 (4) according to equipment in (2), a manufacturing equipment of a single silicon crystal, wherein a heating body is disposed close to the molten liquid surface on the outside of said partition ring. PA0 a) The process is based on the conventional CZ process, where PA0 b) The crucible is separated into two areas, a crystal growing area and a raw materials melting area, by a cylindrical partition made of fused quartz. PA0 a) The generation of solidification at liquid surface on the partition wall facing crystal pulling area (as pointed out in No. 241889/87, page 2, lines 12-16). If the solidification is once generated, the growth continues until the whole surface in the crystal growing area solidifies. PA0 b) Incomplete melting of raw materials. Countermeasures for the above problems
That is, granular silicon floats on the molten silicon liquid surface right above the solidifying point and heat is radiated quickly therefrom as the radiant heat, thus developing solidification around the floating granular silicon. Furthermore, a wave pattern generated when granular silicon is dropped also causes a problem.
On the other hand, there are inventions in the oxide semiconductor field such as those disclosed in Provisional Publication No. 88896/81 and Provisional Publication No. 36997/83. According to these inventions, since the diameter of a pulled crystal is small, a smallsized double crucible may be used, thereby to heat a double crucible directly by induction heating, and thus, solidification of the molten liquid between crucible can be prevented. However, in case of a single silicon crystal, the pulled single crystal has a large diameter and is expensive, and contamination is also caused. Therefore, a metallic crucible cannot be used, but a high quality quartz crucible is usually used. Accordingly, the induction heating system cannot be employed.
Also, according to the invention disclosed in Provisional Publication No. 130195/83, a quartz crucible having a double construction is used, and it looks at first sight that there is no problem against solidification of molten raw material portion. However, as pointed out in a publication described later (Provisional Publication No. 241889/87, page 2, line 12 to line 16 of "Problems to Be Solved by the Invention"), the problem of solidification starting from the contact portion of the inside crucible with the molten liquid surface has not been solved as yet. Moreover, it is conjectured that the area where the molten liquid on the outside of the inside crucible comes in contact with the outside crucible reaches close to 90% of the area where the whole molten liquid comes in contact with the outside crucible in the crucible of a double construction according to the present invention, and majority of the heat from the heater enters directly into the molten liquid on the outside of the inside crucible. Accordingly, it is difficult to raise the temperature in the inside crucible when a single silicon crystal having a large diameter is pulled. In order to raise the temperature compulsorily to a crystal upbringing temperature, and to prevent abovementioned solidification starting from the contact portion of the inside crucible with the molten liquid surface, extensive quantity of heat, viz., heater electric power is required, which is not practical. Furthermore, in this invention, since a feeding pipe of silicon raw material is inserted between the inside crucible and the outside crucible, the silicon raw material is fed as the result, through a feeding pipe immersed in the molten liquid on the outside of the inside crucible. However, the silicon raw material is not molten instantaneously on the molten liquid surface with such a feeding method. Therefore, the silicon raw material reaches a high temperature, but is accumulated in the feeding pipe as it is a solid body. When accumulation once occurs, a sintered state is produced between raw materials and between the raw material and the inner wall of the feeding pipe, and it becomes impossible to supply the raw material thereafter. Because of such reasons, this invention has not been put to practical use as yet.
There are those inventions disclosed in Utility Model Provisional Publication No. 141578/84 and Provisional Publication No. 241889/87 as similar inventions to above-mentioned invention (Provisional Publication No. 130195/83). In the former invention (Utility Model Provisional Publication No. 141578/84), a ring body is floated on the molten liquid. However, according to this equipment, there is a convention of a molten liquid between the single silicon crystal pulling portion and the granular raw material feeding portion, and the temperature on the outside of the floating ring reaches right above the melting point which is almost equal to that of the single silicon crystal pulling portion theoretically. Therefore, the basic problem of progress of solidification from granular silicon floating on the liquid surface has not been solved at all. Moreover, the problem of progress of solidification starting from the floating ring pointed out in the latter specification (Provisional Publication No. 241889/87, page 2, line 12 to line 16 in "Problems to Be Solved by the Invention") is not solved, but only the problem of the wave pattern has been solved.
On the other hand, in the latter invention (Provisional Publication No. 241889/87), there is provided along the outside surface of the crucible a vertical through for feeding the silicon raw material into the crucible via a through hole provided on the crucible. However, the capacity of the raw material melting portion of the vertical through is small. Therefore, when the silicon raw material having a very large fusing latest heat is supplied continuously, the raw material cannot be molten completely. Also, the through hole being near to the molten silicon level, the molten liquid having different density moves straight to the single crystal interface with the convection, thus concentration fluctuation is easily produced and crystal growth with high quality is hindered. In addition, with this invention, very expensive processing of a quartz crucible is required, which causes increase in cost.
Also, according to the invention disclosed in Provisional Publication No. 36197/86, a heat insulating cover is disposed above the molten liquid surface on the outside of the partition (dam) so as to melt granular raw material rapidly. With this invention, however, as pointed out in above-described Provisional Publication No. 241889/87, heat radiation from the partition can neither be controlled, and the problem of generating solidification from the contact portion of the partition with the molten liquid surface has not been solved as yet.
When a single silicon crystal is pulled while feeding granular silicon into the crucible continuously and directly on the basis of above-mentioned conventional technique, there are such problems as follows.